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Inferior Wall M.I.

Tue, 11/17/2015 - 14:38 -- Dawn

This ECG shows a common manifestation with inferior wall M.I., BRADYCARDIA.  We see the signs of acute inferior wall M.I. in the inferior leads:  II, III, and aVF all have ST segment elevation.  There almost appear to be pathological Q waves in Leads III and aVF.  There are still VERY tiny r waves, and the downward deflections are not wide, but should full-blown Q waves develop in these leads, they would signify necrosis in the area.  A repeat ECG would certainly be warranted. 

Another sign that there is an inferior wall STEMI is the ST segment depression in Leads I and aVL, which are reciprocal to Lead III.  ST depression can have many meanings, but when it is localized in the leads which are opposite ST elevation, it is reciprocal.  There is also ST depression in Leads V1 and V2.  These leads are reciprocal to the POSTERIOR wall, otherwise known as the upper part of the inferior wall.  If an inferior wall M.I. is large enough, it can produce ST elevation in the posterior leads (not performed in this case), and ST depression in the anterior leads, especially V1, V2, and V3. 

The rhythm is a marked sinus bradycardia, at just under 40 beats per minute.  Sinus bradycardia is very common in inferior wall M.I., because the inferior wall and the sinus node are usually both supplied by the right coronary artery.  AV blocks can also occur because the AV node is also supplied by the RCA in most people. 

It is important to remember that bradycardia does not always need to be treated.  In patients with acute M.I., a well-tolerated bradycardia may actually be beneficial to the injured heart, reducing supply/demand ischemia.  A well-tolerated bradycardia is a rate that does not produce low blood pressure and poor peripheral perfusion.  Some people tolerate rates in the 40’s quite well.  If the patient shows signs of poor perfusion: low BP, decreased mentation, pallor, shortness of breath, the rate should be cautiously increased with medication or electronic pacing.  

 

Inferior Wall M.I. and Right Bundle Branch Block

Fri, 10/24/2014 - 15:36 -- Dawn

This ECG shows two obvious abnormalities, right bundle branch block AND inferior wall M.I.  It is also a good teaching example of how the terminal wave of RBBB can be mistaken for the ST elevation of M.I.

First, check this ECG to see if it meets the criteria for right bundle branch block:

1)  The QRS will be wide. That is, it will be greater than or equal to .12 seconds (120 ms).  In this case, the QRS is 134 ms.

2)  The rhythm will be supraventricular.  Supraventricular rhythms originate from above the ventricles.  This ECG has P waves before each QRS.  Even though the rhythm is irregular, slowing down during this recorded period, it is a sinus rhythm.

3)  The QRS will have a terminal wave after the "normal" part of the QRS.  This represents the right ventricle depolarizing late.  It is very easily seen in V1, which normally has an rS pattern, and with RBBB has an rSR' pattern, making it appear upright.  V6 and Lead I will show this terminal wave as a wide little s wave.

As mentioned, there is also an acute inferior wall M.I. here.  The ST segment elevation in Leads II, III, and aVF are actually quite subtle.  The flat top of the ST segments gives them away as abnormal, along with the associated ST elevations in V5 and V6, and the reciprocal ST depressions in V1 through V3.  Normally, in IWMI, there will be reciprocal ST depressions in Leads I and aVL, but the elevations they are reflecting are very subtle, and so, therefore, are the depressions. 

The tricky thing about this ECG is that you must look carefully at the inferior wall leads to see the true ST elevation, which, as mentioned, is subtle.  The RBBB terminal wave of the QRS complexes in Leads III and aVF is upright, and is often mistaken for ST elevation.  Remember, ST segments are smooth from the end of the QRS to the peak of the T wave.  See the detail illustration.

This ECG is suitable for your classes from beginner level (rate variation in sinus rhythm) through advanced (clinical significance of RBBB in acute M.I.).  It also offers an example of reciprocal ST changes, and of a situation where the inferior leads II, III, and aVF are related to the low lateral leads V5 and V6 by a shared blood supply.

Inferior Wall MI With Artifact

Thu, 10/16/2014 - 20:11 -- Dawn

This ECG is taken from a 66-year-old man who presented to the Emergency Dept. with a complaint of chest pain.  The ECG shows clear signs of acute inferior wall MI:  ST segment elevation in Leads II, III, and aVF and reciprocal ST depression in Leads I and aVL.  In addition, there are reciprocal ST depressions in Leads V1, V2, and V3.  These indicate that the MI extends up the inferior wall into the area called by most clinicians the posterior wall.  When the injured area extends high enough from the inferior wall, it becomes visible to the anterior-septal leads as ST depression.  There is also a small ST elevation in Leads V5 and V6, the low lateral wall, indicating a common blood supply for the inferior and low lateral walls (usually the right coronary artery).  All of these findings make this a rather "typical" inferior wall MI.

Unfortunately, this ECG also has a significant amount of artifact.  The second, sixth, and tenth "beats"  appear to be  premature beats in Leads I and II.  However, it is important to remember that the four channels on this ECG are run simultaneously.  That is, any complex of significant voltage should show up four times.  The "premature" beats do not appear in Lead III, and do not affect the timing of the appearance of the next beat at all.  They also appear during moments of baseline disruption, indicating that they are not heartbeats, but simply artifact.

Why is this important?  Artifact makes the ECG hard to interpret accurately.  The ECG machine even had a difficult time, completely ignoring obvious P waves, and calling the rhythm "atrial fibrillation".  Every effort should be made to obtain the cleanest, most artifact-free ECG possible.

Additional note:  it can be very informative to do a right-sided ECG on an IWMI patient, or at least a V4Rt.  In fact, it is a protocol requirement in many EMS agencies.  Right ventricular infarction can change the hemodynamics of your patient, causing a need for fluid resuscitation. In fact, a drop in BP, such as that caused by nitroglycerin, can cause circulatory collapse.  Ntg should be given cautiously to RVMI patients.  Fortunately, IV fluids will seldom cause left heart overload in these patients.  A look at the right ventricle with V4Rt can be very helpful in deciding treatment options.

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